I dated this guy back in high school who was a cheater. I had a feeling he was messing around with other women, but I was in denial until one of his exes found me on myspace and suggested I get checked. Apparently, he'd cheated on her (when they'd been dating) and ended up giving her chlamydia.
Thankfully, the one time during our short-lived relationship that we had sex, we'd used a condom, so my STD panel came back negative for everything.